The present invention relates to lasers, and more particularly to a method and device for stabilizing the emission wavelength of a laser source, in which this stabilization is achieved by locking to a reference wavelength which is very different from that which is to be stabilized.
Preferably, but not exclusively, the invention is applied to the stabilization of the emission wavelength of a semiconductor laser used as the source in an optical communication system.
It is Known the emission wavelength of lasers is subject to fluctuations, and therefore the lasers are associated with control circuits which detect the deviation from the nominal value and generate an error signal which is sent to the laser operating devices to keep the emission wavelength at this nominal value.
In a commonly used stabilization method, the emission wavelength of the laser is locked to an atomic or molecular line, e.g. an absorption line of a gas which has an absorption spectrum with lines whose wavelength is close to the emission wavelength of the source. An example of a locking method of this type for the application, preferred here, to the field of optical telecommunications, is described in European Patent 0 660 470. This stabilization is absolute, in other words independent of the environment, and provides the source with good short- and long-term stability and good reproducibility of the emission wavelength.
In general, however, atomic or molecular lines close to those used for optical telecommunications are relatively wide and therefore the stabilization achieved may be insufficient. For this reason, numerous proposals have been made in the literature for the provision of locking on absorption lines at wavelengths relatively distant from that of the source to be stabilized, which have more advantageous characteristics for the intended purposes. An example is that of an absorption line of rubidium at 780 nm. Absorption lines at this wavelength are used for the purposes of stabilization in various fields, including those which are very different from telecommunications (e.g. metrology).
However, it is generally difficult to produce wavelength comparison systems for signals having very different wavelengths from each other, and therefore attempts are made to bring the wavelengths for comparison closer to each other.
The article xe2x80x9cAbsolute Frequency Control of a 1560 nm (192 THz) DFB laser locked to a Rubidium Absorption Line Using a Second-Harmonic-Generated Signalxe2x80x9d, by C. Latrasse and others, IEEE Transactions on Instrumentation and Measurement, vol. 44, no. 4, August 1995, pp. 839 ff., describes a system in which the radiation at 1560 nm emitted by the laser to be stabilized is directed into a crystal of KNbO3 which generates its second harmonic (corresponding to a wavelength of 780 nm). This second harmonic is separated from the main signal at 1560 nm and sent to a cell containing 87Rb for interaction with the line at 780 nm. The signal leaving the cell is subsequently detected in a silicon photodiode to produce the error signal to be supplied to the laser control devices.
Other methods, such as those described in the articles xe2x80x9cWide-Span Optical Frequency Comb Generator for Accurate Optical Frequency Difference Measurementxe2x80x9d, by M. Kourogi and others, IEEE Journal of Quantum Electronics, vol. 29, no. 10, October 1993, pp. 2693 ff., and xe2x80x9cGeneration of Frequency-Tunable Light and Frequency Reference Grids Using Diode Lasers For One-Petahertz Optical Frequency Sweep Generatorxe2x80x9d, vol. 31, no. 3, March 1995, pp. 456 ff., produce the stabilization by locking the laser source to be stabilized to a highly stable reference source. For this locking, optical signals with a frequency equal to the sum of and/or the difference between the frequencies of the two sources are generated, and the beat signals are detected. These beat signals are then fed back to act on the laser to be controlled.
In all these known methods, the creation of the error signal requires two separate operations: the first is the generation of an optical signal at a suitable frequency by the generation of a harmonic of the signal to be stabilized (in the case of the first document cited) or the mixing of this with a reference signal; the second operation is the detection of the converted signal. This makes the corresponding equipment complicated and makes the process inefficient.
The object of the invention is to provide a method and equipment which enable this disadvantage to be overcome.
According to the invention, a method is provided for stabilizing the emission wavelength of an optical source which emits coherent radiation at a first wavelength, by locking to the emission wavelength of an optical reference source, which emits coherent radiation at a second wavelength which is substantially different from the first, in which method the radiations at the first and second wavelengths are made to interact an optically electrical signal resulting from the interaction is detected, a feedback signal is produced from the detected signal, and the said interaction is represented by a two-photon absorption for the first wavelength, which gives rise to a beat signal having a frequency equal to the difference in frequency between a radiation at a wavelength which is half the first wavelength and the second radiation, this beat being detected concurrently with its creation.
The invention also relates to the device for implementing the method.
The phenomenon of two-photon absorption is a non-linear phenomenon based on the fact that two photons which are coherent in phase can interact to excite an electron in a semiconductor material to an energy twice that of a single photon. Owing to this phenomenon, a coherent radiation to which the material would be transparent can be absorbed, thus generating electron-hole pairs. These can then be detected as a photodetection current. A fuller description of the phenomenon can be found, for example, in xe2x80x9cOptical processes in semiconductorsxe2x80x9d by J. I. Panhove, Dover Publications, Inc., New York, USA, 1971: see, in particular, Sections 12-A-4 and 12-A-5 on pp. 268 ff.